1. Field of the Invention
The present invention relates to a method of manufacturing an optical fiber cable covered with a metal pipe, and an apparatus for manufacturing this optical fiber cable.
2. Description of the Related Art
When a tension is applied to an optical fiber having a diameter of 250 .mu.m, although a breaking strength is as rather large as about 6 kg, the elongation is 3 to 6%. This elongation is considerably small as compared to that of a conventional copper or aluminum cable. For this reason, a tensile strength member must be provided to the optical fiber to maintain a high optical fiber cable strength. When an optical fiber is dipped in water, its strength is sometimes degraded. Therefore, when laying an optical fiber cable underseas or underwater, an optical fiber cable having a sheath structure obtained by covering an optical fiber with a thin metal pipe must be employed to maintain a high laying tension and water resistance.
Conventionally, when such an optical fiber having a small diameter is to be covered with a metal pipe, the optical fiber is inserted in a metal pipe having a gap in the longitudinal direction, and this gap is welded by soldering. According to this method, however, heat generated during welding of the metal pipe is applied to the optical fiber through the gap for a comparatively long period of time, leading to thermal damage to the optical fiber.
Jpn. Pat. Appln. KOKAI Publication No. 64-35514 discloses an apparatus and method of continuously manufacturing an optical fiber covered with a metal pipe by welding the abutting portion of the metal pipe with a focused laser beam, so that thermal damage in the optical fiber is prevented. In this apparatus for manufacturing an optical fiber cable covered with a metal pipe, a flat metal strip which is continuously supplied is formed into a metal pipe having a longitudinal gap at its top portion. An introducing tube is inserted in the metal pipe through the gap in the metal pipe, and an optical fiber is inserted in the metal pipe through the introducing tube. After the gap of the metal pipe in which the optical fiber is introduced is closed, the metal pipe is supplied to a laser welding unit. The laser welding unit irradiates a laser beam having a focal point at a position outwardly remote from the surface of the abutting portion while positioning the abutting portion at the top portion of the supplied metal pipe with a guide roller, thereby welding the abutting portion. In this manner, welding of the abutting portion is realized by shifting the focal point of the laser beam from the abutting portion without protecting the optical fiber with a heat-shielding member. Subsequently, the outer diameter of the metal pipe incorporating the optical fiber is reduced to a predetermined size, and the metal pipe is wound on a capstan and continuously withdrawn from the capstan.
In withdrawal of the metal pipe, an inert gas is supplied to the introducing tube. The optical fiber is transported into the metal pipe with the viscosity resistance of the inert gas. While the metal pipe is engaged with the capstan, the optical fiber is positioned on an outer side of the inner portion of the metal pipe by blowing the inert gas. Thus, when the metal pipe is set straight, the length of the optical fiber becomes larger than that of the metal pipe, so that the optical fiber flexes in the metal pipe, thereby preventing the optical fiber from causing a strain by the laying tension or the like.
Furthermore, when the metal pipe is damaged to form a hole, water can enter through the hole to degrade the optical fiber. In order to prevent this, a gel filler is injected into the metal pipe. More specifically, after the optical fiber is blown to the outer side within the metal pipe with the inert gas at the capstan, a filler is injected into the metal pipe through a filler introducing tube which is different from the introducing tube that introduces the optical fiber.
However, in the conventional method of manufacturing an optical fiber cable covered with a metal pipe, since the position of the introducing tube is not constant in welding the abutted portion of the metal pipe, the introducing tube is sometimes arranged close to the abutted portion of the metal pipe. In this case, heat generated by laser welding is applied to the optical fiber to thermally damage it. For example, when the temperature in laser welding is measured while the optical fiber is close to the abutted portion in this manner, the temperature near the optical fiber is increased to at least about 600.degree. C. Thus, a crystallite nucleus is formed in the optical fiber. As the crystallite nucleus grows, it increases the scattering loss, leading to a disadvantage. This is a phenomenon called devitrification.
During welding, a spatter is generated. When the introducing tube is close to the abutted portion, the spatter deposited on the introducing tube tends to contact the bead on the rear surface of the welding portion. When the spatter contacts the rear surface of the welding portion, the heat balance of the welding portion is disordered to cause defective connection. When formation of the rear bead portion that generates the spatter is excessively suppressed in order to avoid defective welding caused by the spatter, a non-welded portion is undesirably formed. Also, when the introducing tube is in contact with the abutted portion, a phenomenon similar to defective welding caused by the spatter occurs. Therefore, it is impossible in practice to perform a long-time continuous manufacturing operation, i.e., to perform an operation of manufacturing an elongated metal pipe-covered optical fiber cable.
Furthermore, in laser welding, if the irradiating power intensity of the laser beam is excessively high, the metal pipe is heated to a temperature equal to or higher than its boiling point, so that part of the metal pipe is evaporated or scattered to form a hole in the irradiated surface. In order to prevent this, conventionally, the focal point of the laser beam is not formed within the thickness of the abutted portion of the metal pipe but is formed above the metal pipe. When, however, the focal point is formed above the metal pipe, the sectional shape of the melt zone swells in its lower portion. Then, a shrinkage void or crack tends to be formed in the metal pipe, and the amount of spatter is increased.
From these situations, one of the present inventors previously proposed a method disclosed in U.S. Pat No. 5,241,153 and U.S. Ser. No. 08/078,574 to solve the above problems. According to this proposal, the focal point of the laser beam emerging from the laser irradiating means is shifted by fixing the abutted portion of the metal pipe at a predetermined position slightly above or below the focal point of the laser beam and/or the path line of the metal pipe, and the laser beam is irradiated in this state.
The present inventors noticed the fact that the optimum width of the opening formed in the abutted portion of the metal pipe differs depending on other welding conditions (the welding rate, the power of the irradiated laser beam, and the focal shift amount of the laser beam), and studied an apparatus for adjusting the opening width of the abutted portion of the metal pipe. In the process of the study to reach the present invention, the present inventors considered adjusting the opening width of the metal pipe with known squeeze rolls. However, since the diameter of the metal pipe is too small, even if the metal pipe is held by the known squeeze rolls, the held portion and the welding portion are separated. As a result, the gap is gradually increased due to the function of spring back of the metal pipe. Thus, it is difficult to appropriately adjust the opening width of the abutted portion at the welding portion. More specifically, when the opening width is increased, welding becomes unstable, so that the welding rate must be decreased. On the contrary, when the opening width is excessively small, defective welding occurs. Thus, the welding rate cannot be increased in the same manner as in a case wherein the opening portion is excessively widened.